1. Field of the Invention
The invention relates to ceramic cold conductors and somewhat more particularly to ceramic cold conductors based on barium titanate and methods of producing such conductors.
2. Prior Art
Ceramic cold conductors (sometimes referred to as ceramic PTC) generally are comprised of barium titanate, which in itself, when sintered in an oxidizing atmosphere, is a high ohm dielectric. However, with the additions of appropriate doping elements, such as for example, antimony, lanthanum, yttrium and other rare earth metals, such barium titanate can be made conductive for use as a ceramic cold conductor. Specifically, such ceramic PTCs exhibit a substantially large increase of specific electrical resistance for such material in a relatively narrow temperature range, that is at the Curie temperature of the material, which is known for such cold conductors as the resistance with a positive temperature coefficient (PTC).
With appropriate doping, attainable specific cold resistance values for a given ceramic cold conductor material is at least about 10 ohm.cm because higher dopant concentrations cause increases in the resistance. This phenomena, as was established, is connected with the prevention of grain growth during sintering by the presence of a relatively high amount of doping elements. Even in such highly doped barium titanate, there arises at high temperatures, barium vacancies which can compensate for the effect of the doping and which are enriched at the grain boundaries so that high ohm surface layers or zones develop in individual crystallites. These surface layers are necessary, on the one hand for the actual cold conductance effect, however, on the other hand, in instances of grain size below about 1 .mu.m, such surface layers extend over the total grain volume so that no conductivity at all can occur. Further details of this phenomena can be derived from the literature, for example, from "Siemens Forschungs-und Entwicklungsberichte", (Siemens Research and Development Reports), Vol. 8, No. 4, (1979), pages 209 et seq.
In many applications, cold conductors have a low degree of cold resistance are desired. Cold resistance is defined as the resistance value at a normal operating temperature, for example, room temperature or 25.degree. C., that is, at a temperature far below the reference, or Curie temperature. Such low resistance values were attained heretofore the individual cold conductor bodies by providing such bodies with correspondingly larger disc surfaces and smaller thicknesses. However, such conductor body shapes require a substantial increase of space.